In coal mining, electrical control equipment is generally located near the working coal face. Such electrical equipment must be located within a protective enclosure of "explosion-proof" design to prevent electrical sparks or arcs from igniting any explosive mixture of methane and air which may be found in the area. In addition, methane monitors sense the presence of methane in the general atmosphere and shut off the power if a safe threshold level is exceeded. The mine ventilation system ideally also prevents an accumulation of methane gas which would be sufficient to cause an explosion. Short circuit protective devices are designed to prevent the occurrence of a prolonged arc. Such safety measures provide redundant protection, so that if one or even two should fail, there is still some degree of safety to depend upon.
Each of these presently used methods of protection has some mode of failure. For example, methane monitors are subject to being tampered with and hence disabled in various ways. Short circuit protective devices sometimes fail to interrupt faults which cause arcing. The high temperature pressure switch assembly of the present invention offers an additional approach to ensuring safety in hazardous locations, the need for which will be described in further detail in the following paragraphs.
These explosion-proof enclosures are of very rugged construction as a result of strictly controlled design criteria, and are government tested at a specified pressure level. Presently, enclosures must be designed and built to withstand an inside static pressure of 150 psig, and incorporate typically 1/2 inch thick walls. Flame-quenching joints are provided typically by 1 inch flange widths, and 0.004 inch maximum flange gaps. The enclosure must retain its explosion-proof qualities after being subjected to a battery of internal pressures.
Testing has shown that although it is unlikely, methane can leak into the interior of an enclosure, and result in an explosive mixture of methane and air, even where the gap between the enclosure flange and its cover is very small. A normally occurring spark can ignite these gases. Fundamental to the requirement that the enclosures be of "explosion-proof" design is that any internal ignition or explosion must be contained within the enclosure and must not propagate to an external explosive atmosphere.
In addition to the occurrences of explosions inside the enclosure, other significant sources of atmospheric pressure increase exist, such as that resulting from the thermal effect of a persistent electrical arc, and that from chemical pyrolysis of the electrical insulating material.
The magnitudes of pressures which may occur inside an enclosure are related to the following:
1. The magnitude of pressure which can result from a persistent electrical arc is directly related to the power available in the circuit and the amount of "free volume" inside the enclosure. The thermal effects of an electrical arc which is not interrupted by the circuit protective device will cause the pressure to increase steadily over the fault duration. PA1 2. Laboratory testing has demonstrated that an explosion of methane and air may produce a pressure of up to approximately 100 psig. The occurrence of such an explosion which is a result of an electrical arc as described above, may result in combined (additive) pressures in excess of the pressure holding capability of the enclosure. PA1 3. Pyrolysis is a chemical decomposition of electrical insulation material. Where the insulation material between terminals is contaminated with dust or moisture, a phenomenon called "tracking" occurs. Voltage stresses cause minute amounts of current to flow along the insulation surface. These currents gradually increase (over a long period), and some erosion of the insulation material may occur. If the circuit protective device fails to sense the fault, the process eventually reaches a point where a power arc is established along the surface of the insulation material. The intense heat from the arc causes chemical decomposition of certain widely used insulation materials (such as phenolic resins) and the generation of large quantities of various flammable or toxic gases, including highly explosive hydrogen. In a closed vessel such as an explosion-proof enclosure these gases cannot escape and very high pressures (much higher than the 150 psig design strength) can be developed. These severe pressure increases can be prevented if the power is quickly shut off when the pressure is just beginning to rise. PA1 "the major disadvantage of pressure sensors was that we were unable to find one which would be certain of operating properly at the elevated temperatures which might also be expected." PA1 1. The high temperature pressure switch assembly must be sensitive to sudden, but relatively low increases in atmospheric pressure, and its response time must be fast enough to remove power from the circuit before the pressure inside the enclosure is at a critical level. In an experimental proving test of the pressure switch assembly of this invention it was placed inside an enclosure containing a stoichiometric mixture of methane and air, which was ignited by an electric spark at a known instant. Pressure/time recordings indicated that the pressure switch assembly actuated on a pressure increase of less than one psig which occurred 0.035 seconds (2.1 cycles at 60 Hz) after ignition. The remote circuit breaker opened 0.050 seconds (3 cycles) after ignition, by which time the pressure had increased to only 10 psig. PA1 2. The pressure switch assembly must be resistant to the possibly high temperatures and pressures encountered in explosions, electric arcs, and pyrolysis events. This depends upon the material of the diaphragm or other pressure-movable member, since it must deform or move under relatively low atmospheric pressure increases (hence, in the case of a diaphragm, it must be very thin). The actuating diaphragm 96 and the auxiliary, heat protective diaphragm 97 of the embodiment illustrated here are 5 mil DuPont "Kapton" plastics film. This material has a tensile strength of 24,000 psi at 23 degrees Centigrade, and a 400-degree Centigrade rating (2-hour test). PA1 3. The pressure switch assembly must not be a cause of "nuisance" power outages, and hence must not be accidentally operated by vibration. The force required to actuate it must greatly exceed that which results from vibration, eliminating any such concerns. PA1 4. The pressure switch assembly should not itself be a source of sparks or arcs, or introduce any new safety hazard to the electrical system. It can utilize either sealed contracts, which eliminate any incendive sparking when a signal is sent, or can be made explosion-proof itself. PA1 5. The pressure switch assembly should be tamper-resistant, and not easily disabled. Its rugged construction (housing walls of this embodiment are nearly 1/2 inch thick) and double top cover with wide flange seals eliminate these concerns. PA1 6. The pressure switch assembly should be of the "latching", or "manually resetting" type. Maintenance personnel must remove the cover of the enclosure and manually reset the pressure switch assembly before power can be restored to the circuit after it is shut off from its power supply, providing opportunity to investigate and correct the cause of a pressure increase which has tripped the switch. PA1 7. The actuating diaphragm or other pressure-movable member should be of relatively large area to provide an effective operating force with a small pressure differential. In an embodiment tested, the diaphragm area was 7.07 square inches. A pressure of only 0.85 psig on the diaphragm provided approximately 6 pounds of force, which tripped the pressure switch assembly. Should the tripping/latch mechanism become dirty or "sticky", tripping would require a slightly higher pressure. This ability to produce a large change in force in a very short time also gives the pressure switch an advantage over methane monitors which must be precisely calibrated to detect only a 2 percent concentration of methane.
Very high temperatures can be expected in an enclosure from a methane explosion, an electrical arc, or pyrolysis of electrical insulation. This has discouraged development of pressure-sensing switches, prior to the present invention, because of the widespread belief that they would not operate properly at the high temperatures. For example, a February 1986 mining research contract report, "Development of High Voltage Permissible Load Center", summarizing a 5-year research effort by Foster-Miller, Inc., stated,
Coal mining technology continues to advance rapidly and the power requirements of mining machines continues to increase. Motor sizes have increased and so have utilization voltages. As the capacity of power systems increase, it becomes more likely that pressures occurring in enclosures resulting from explosions, arcing or pyrolysis may be greater than the presently required 150 psig design capacity. In addition, higher utilization voltages increase the likelihood of tracking failures causing chemical pyrolysis. Properly installed inside an explosion-proof enclosure, the high temperature pressure switch assembly of the present invention can trip a remote circuit breaker, thus cutting off the power supply to the enclosure. This pressure switch assembly satisfies the need to maintain, or even possibly increase the degree of safety by diminishing the chances of enclosure failure due to excessive pressures, despite these upward trends in motor sizes and utilization voltages.